4-Phenyl (CAS NO.: ), with other names as Semicarbazide, 4-phenyl-, could be produced through the following synthetic routes.

In a 500-cc. round-bottomed flask fitted with a reflux condenser are placed 68 g. of phenylurea (0.5 mole) and 120 cc. (1 mole) of 42 per cent hydrazine hydrate solution. The flask is heated on a steam bath for about twelve hours. The hot mixture is treated with a small amount of decolorizing charcoal (Norite) and filtered. The charcoal is washed with two 15-cc. portions of warm water, and the filtrate and washings are then concentrated on a steam bath to about 100 cc. On cooling in an ice bath a crop of crystals separates which is collected on a filter and washed with two 15-cc. portions of cold water. The filtrate and washings are concentrated to about 25 cc., and another crop of crystals is obtained as before. The total yield of crude compound is 47–52 g. It is white at first but sometimes turns brown on drying. It usually melts below 115° because of some unchanged phenylurea.

The product is purified by conversion to the hydrochloride, which is then changed into the free base.

A filtered solution of the crude product in 200 cc. of hot absolute alcohol is treated with 250 cc. of concentrated hydrochloric acid. Most of the hydrochloride precipitates at once and is filtered, washed with alcohol, and dried. The filtrate is cooled in an ice-salt bath and again filtered, and the precipitate is washed, dried, and added to the previous portion. The yield is 46–48 g. of material melting at about 215°. The hydrochloride is dissolved in three times its weight of water, previously heated nearly to boiling. The solution is filtered if necessary and then treated with sodium hydroxide solution (2.2 g. of 10 per cent sodium hydroxide solution for each gram of hydrochloride used). The free base separates at once, and the solution is cooled in an ice bath and filtered. This product melts at 120–123°. The yield of pure base is 28–30 g. (37–40 per cent of the theoretical amount).